Add set_qflist and set_loclist.
- Also add locations_to_items, which calculates byte offsets for
character positions in files and avoids unnecessary operations.
- Implements textDocument/formatting, textDocument/rangeFormatting,
workspace/applyEdit.
TODO:
- still has edge cases around replacement probably. Only tested with
inserts on the same position.
- Rename builtin_callbacks to default_callbacks and slightly change its
semantics:
- No longer contains the default implementations. Instead, any
default_callbacks will be used in preference for our .buf methods.
- Add this to the docs.
